[Chapter V] Basic Knowledge of Discrete Semiconductor Devices

© 2015, Toshiba Corporation

Basic Knowledge of Discrete Semiconductor Devices

Chapter V Optical Semiconductor Devices • LEDs • Photocouplers

March, 2015 Semiconductor & Storage Products Company

Toshiba Corporation

2 © 2015, Toshiba Corporation

Semicon-ductors to transform electricity

to light

Semicon-ductors to transform

light to electricity

Photodetectors

IR

UV

Current

A

Current

Light-emitting devices

Visible LEDs Devices that emit visible light, ranging from violet to red and white etc.

Photodetectors Photodiodes, photosensor ICs, etc. A group of products that output electric signals in response to change of light

Fiber couplers They transform electricity into light and vice versa for communication using optical fiber.

Photocouplers Light-emitting devices and photodetectors combined in the same package. These products transfer electric signals while maintaining electric isolation.

From the next page onward, LEDs and photocouplers are explained.

Types of Optical Semiconductors

The types of optical semiconductors are as follows: (1) Light-emitting devices: visible LEDs, infrared LEDs, ultraviolet LEDs, laser diodes (2) Photodetectors: photosensors, solar batteries, CMOS sensors (3) Combination devices (light-emitting devices with photodetectors): photocouplers, fiber

couplers


An LED (Light-Emitting Diode) emits light by forward current flowing to PN junction of a compound semiconductor. Carriers (electrons and holes) move by forward current flowing to the LED. Holes move from P layer to N layer, and electrons move from N layer to P layer. These injected carriers are recombined in each layer. The difference between the energy before recombination and the energy after recombination is emitted as light. Emitted light varies depending on the energy gap (Eg) of compound semiconductor. (Note: Generally, Si diodes do not emit light because recombined energy is converted to heat.)

P type N type

Holes

Light Light Electrons

Recombination

Eg

Light Emitting Principal of LEDs (Light-Emitting Diodes)

Recombination


An LED emits ultraviolet, visible and infrared light at various wavelengths. The wavelength is expressed by the following formula using the material’s energy gap (Eg). λ (nm)=1240/Eg(eV) Materials with larger Eg emit shorter wavelength light, and materials with smaller Eg emit longer wavelength light. Infrared LEDs used for remote controllers of TVs etc. use GaAs (gallium arsenic). LEDs for red and green indicators use GaP or InGaAlP and blue LEDs use InGaN or GaN.

Material Energy gap Eg @300K (eV) Wavelength λ(nm) Color

GaAs 1.4 885 Infrared

GaP 2.26 549 Green to red

InGaAlP 1.9 to 2.3 539 to 653 Green to red

InGaN 2.1 to 3.2 388 to 590 Ultraviolet to green

GaN 3.4 365 Ultraviolet to blue

紫外

LED Colors

Blue Green Green Red Red

UV Purple

Blue Green

Yellow

Orange

Red IR

Com

para

tive

light

in

tens

ity

Dashed line is visual sensitivity curve

Wavelength (nm)


The left-hand figure shows an example of the basic structure of an LED chip assembled in a frame. The LED chip, having P layer grown on N substrate, is bonded with conductive silver paste to the frame, and by gold wire to the electrode. The right-hand figure shows an example of the structure of an LED chip assembled in an SMD (Surface Mounted Device) package. When the shape of a transparent resin package is flat as in the figure below, the light emission pattern is wide. In the case of a lens-shaped transparent resin package, the light emission pattern has high directivity.

Example of an LED chip structure

Ｐ layer

N substrate Emission area

Au fine wire

Frame

LED chip

Conductive silver paste

Anode electrode Cathode electrode

Package resin

Transparent package resin

Au fine wire

Structure of LEDs

Example of an SMD LED structure


Mixing red, green and blue.

Mixing blue and yellow.

White can be made by mixing three primary colors (R,G,B)

Realization of blue LED made white LED

possible.

R

G

B B

Y

White light is made by mixing colored light. • Red(R)+green(G)+blue(B) … Mixing three primary colors makes white. • Yellow(Y)+blue(B) … Adding blue to yellow made by mixing red and green makes white. The figures below are chromaticity diagrams. A chromaticity diagram includes all colors made from RGB three primary colors. Any color (Cx, Cy) can be represented by coordinates.

Principles of White LEDs


• White is made by mixing three primary colors simultaneously emitted from red(R), green(G) and blue(B) LEDs. Color rendering property is superior because of wavelengths of RGB.

• Various colors can be made by adjusting emissions of three colors separately (ex: PWM: Pulse Width Modulation).

• White is made by coating a blue LED with yellow phosphor.

• With low composition of red light, color rendering index is low, around 70.

• Advantageous for high brightness

* Rendering index (Ra): Index of color appearance (reproducibility). The closer Ra is to 100, the better the color reproducibility.

RGB 3 Colored LED

Blue LED + yellow luminescent materials

• White is made by a blue LED coated with red and green phosphors.

• Rendering index is high, around 90. Used for applications that require high reproducibility.

• Somewhat problematic for high brightness.

Methods for emitting colored light to obtain white light are as follows: Using LED that directly emits colored light Using luminescent materials

Mechanism of White LEDs (Light Emission by LEDs and Phosphors)

Blue LED + red + green luminescent materials


Terms Symbol Description

Luminous flux F It indicates amount of light. Total amount of light emitted from light source per second

Luminous intensity Iv It indicates intensity of light, which is luminous flux emitted in a certain direction within a unit solid angle. “Maximum luminous intensity” or “central luminous intensity” are used.

Dominant wavelength λd It indicates the wavelength of light perceived by the human eye.

Peak wavelength λp It indicates the wavelength at which the LED has the maximum light intensity.

Wavelength characteristics

― It indicates relative light intensity corresponding to wavelength. It is also called wavelength spectrum.

Chromaticity Cx/Cy It indicates xy coordinates of the emitted light.

Color temperature CCT It corresponds to the color emitted when burning an object (black body) at a certain temperature.

Directivity characteristics

― It indicates the ratio of luminous intensity at the point of inclination of angle θ when axial luminous intensity of the LED is 100%.

Half-intensity angle 2θ1/2 It indicates the angle between the two points where luminous intensity becomes 50% of axial luminous intensity with regard to directivity characteristics.

Rendering index Ra It indicates the property of a light source that affects how the color of an object appears.

Various terms indicate the optical characteristics of LEDs. They are listed below. Luminous flux, color temperature, rendering index and chromaticity are typical characteristics of white LEDs.

Terms of Optical Characteristics of Visible LEDs


Luminous flux: Total amount of light emitted in 1 second (Unit: lm lumen)

Luminous intensity: Amount of light emitted on a unit solid angle (Unit: cd candela)

Integral ball Detector

Measuring all light with the same intensity by multiple reflection on inside wall of integral ball.

Detector (Light receiving area: 100mm2）

Measuring only the light with visual angle of about six degrees on the axis.

Test condition shown in the right figure is based on Condition B of CIE regulations

Luminous flux F(lm) is the total amount of light. On the other hand, luminous intensity IV(cd) is the amount of light emitted in a certain direction. In the case of using white LED for the lighting application, luminous flux F(lm) is a more important index than luminous intensity. In addition, the “lm/W” index indicates the emission efficiency corresponding to input power.

Optical Characteristics of LEDs: Luminous Flux and Luminous Intensity

100mm


Color Temperature（K） 15,000

10,000

8,000

6,000

5,000

4,000

3,000

2,000 Sunrise, sunset

Cloudy sky

Sunny sky

Sunlight at noon

Daylight color lamp

Natural white lamp

White lamp

Light bulb color lamp

Color temperature is one of the indexes indicating the tone of a color. Its unit is K (Kelvin). Color temperature of an incandescent lamp is about 3000 K, while that of a fluorescent lamp is about 4500 K. Low color temperature makes red light, whereas high color temperature makes bluish-white light. Color temperature of a white LED changes according to distribution of phosphors. For example, when the red emission ratio increases, the color temperature of the white LED becomes low.

Optical Characteristics of LEDs: Color Temperature

Warm white lamp

Candle flame


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

400 450 500 550 600 650 700 750 800

波長（nm）

相対

強度

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

400 450 500 550 600 650 700 750 800

波長（nm）

相対強度

High rendering AAA fluorescent lamp High rendering white LED

Ra: 89 to 99 Ra: 90 Ra: 60

Rendering index indicates how faithfully the light expresses the color of the object. This index is expressed as Ra. Appearance of objects when illuminated by reference light is defined as 100. The higher the rendering index, the more vivid the color becomes. In a light spectrum, peak shapes vary depending on Ra. Generally, in the case of white LEDs, Ra becomes low when light emission efficiency is prioritized.

Two peaks in this area One peak

White LED according priority to emission efficiency

Optical Characteristics of LEDs: Rendering Index R

elat

ive

ener

gy (%

)

Wavelength (nm) Wavelength (nm) Wavelength (nm)

Rel

ativ

e in

tens

ity

Rel

ativ

e in

tens

ity


CIE chromaticity coordinates

2500

3000 4000

5000

Cx

Cy

Colors made by mixing three primary colors are represented by coordinates. This diagram shows chromaticity coordinates (chromaticity chart). All colors including white can be represented by coordinates(Cx, Cy) The white area is around Cx=0.33, Cy=0.33

Example of representing color temperature(K) by chromaticity coordinates. As color temperature increases, color changes successively to red, orange, white and blue.

G

B

R

Optical Characteristic of LEDs: Chromaticity


When making LED emit light by using a resistor to restrict current, forward current changes depending on LED’s forward voltage. Using typical forward voltage, resistor is (5 V-2.85 V) /350 mA = 6.1 Ω. Adopting resistor of 6.1 Ω Case of minimum forward voltage: (5 V-2.7 V) /6.1 Ω=377 mA Case of maximum forward voltage: (5 V-3.3 V) /6.1 Ω=279 mA

(a) Parallel connection The circuit depicted in 1. above is connected in parallel. (2.85 V×350 mA)×3=2.99 W Power consumption of resistor=2.2 W Total power consumption=5 V×350 mA×3=5.25 W * In the case of parallel connection, be sure to connect current limiting resistor on each node. (b) Series connection Voltage must be high, but total power consumption becomes low because of low current. (2.85 V×350 mA)×3=2.99 W Power consumption of resistor=1.15 W Total power consumption=12 V×350 mA=4.2 W

1. Drive by a resistor

E.g.) Characteristics of a white LED

Item Condition Min Typ. Max Forward Voltage(VF) IF=350 mA 2.7 V 2.85 V 3.3 V

LED

5V

GND 1.35 times current difference 1.35 times brightness

difference between the two conditions

6.1Ω

2. Drive multiple LEDs only by resistors.

LED

12V

GND 9.4Ω LED LED

LED

5V

GND

LED

LED

(b)

(a)

Series connection is advantageous

for high voltage.

VF

6.1Ω

6.1Ω

6.1Ω

Explained below are basic driver circuits of Toshiba’s white LEDs with constant voltage source and resistor. Circuit designers should take variation of VF, tolerance of resistor and source voltage, change of temperature, etc. into consideration.

Parallel connection for

low voltage

Example of LED Driver Circuits


p electrode

n electrode

p-GaN

n-GaN

InGaN emission layer

Sapphire(Al2O3) substrate

InGaN emission layer

n electrode n-GaN

p electrode

p-GaN

Silicon(Si) substrate

Comparison of device structures

Structure of a conventional device Structure of Toshiba’s white LED

Toshiba focuses on “GaN-on-Si white LED” whose light emission layer is GaN grown on Si substrate. For conventional LEDs, GaN is usually grown on sapphire substrate. Compared with the conventional approach, GaN-on-Si technology has the following advantages. • Superior cost effectiveness because Si substrate, the mainstream semiconductor material, is used. • Use of large wafers raises productivity.

White LEDs adopting GaN-on-Si Technology


Optic signal

Light

0 1 0

Electric signal

0 1 0

Electric signal

0 1 0

LED

Vcc

Output

Photodetector (Phototransistor)

Metal frame

Outer mold resin (black) Inner mold resin (white) Photocoupler: A photocoupler is a device incorporating a light-emitting diode (LED) and a photodetector in one package. Unlike other optical devices, light is not emitted outside the package. The external appearance is similar to that of non-optical semiconductor devices. Although a photocoupler is an optical device, it does not handle light, but handles electric signals. Examples of a photocoupler’s operation: (1) The LED turns on (0⇒1). (2) The LED light enters the phototransistor. (3) The phototransistor turns on. (4) Output voltage changes 0⇒1.

(1) The LED turns off (1⇒0). (2) The LED stops light emission to the phototransistor. (3) The phototransistor turns off. (4) Output voltage changes 1⇒0.

* The cutaway image on the right shows a transistor output photocoupler with transmissive double-mold structure.

What is a Photocoupler?


Inverter Application

DC powered (5V, 3.3V, etc.)

AC powered (100V, 200V, etc.)

In a photocoupler, input (LED) and output (photodetector) are electrically isolated. Thus, input signal can be sent to output correctly even if electric potentials are different between input and output. In the inverter application shown in the figure on the right, controlling parts such as microcontrollers are usually driven by low DC voltage. On the other hand, IPMs and IGBTs drive loads that need high voltage such as 200 V AC. The microcontrollers can control IPMs and IGBTs by using photocouplers. * Photocouplers to meet various output needs are available.

Why Are Photocouplers Necessary?

M HVIC1

HVIC2

HVIC3

LVIC

IPM

Feedback

Isolation IPM

Driver coupler

Micro- controllers

ASICs

IPM Driver coupler

IPM Driver coupler

IPM Driver coupler

IPM Driver coupler

IPM Driver coupler

Transistor coupler


Products in which LED is

antiparallel connected to

support AC input are available.

Multi-channel single-package products (2 circuits and 4 circuits)

are available.

An LED is used for input of the photocoupler. On the other hand, various devices are used for output. Transistor output A phototransistor is a detector. Darlington type is also available. IC output This product uses a photodiode as detector, and a logic IC or a high-current IC to drive gate of IGBT or MOSFET for output. Triac/Thyristor output A photothyristor or a phototriac is used for output. They are mainly used for control of AC line. Photorelay (MOSFET output) MOSFETs are controlled by photovoltage (photodiode array) that drives MOSFET gate. By this operation, a photorelay can be used as MOSFET output relay like a switch.

Types of Photocouplers (Functions)

Transistor output IC output

Photorelay

Functions

Thyristor/Triac Photovoltage output

Transistor output Darlington transistor output Logic output

Photorelay

Gate drive

Triac output Thyristor output Photovoltage output


沿面距離

空間距離

内部絶縁物厚

AC/１分間

絶縁耐圧

Photocoupler package shapes and isolation voltage must be in accordance with safety standards. Therefore, there are restrictions on package design of photocouplers, unlike other semiconductor products. Creepage distance The minimum distance between two conductors (input-output) along insulator Clearance The minimum distance between two conductors in the air Insulation thickness The minimum distance between two conductors through insulator Isolation Voltage Isolation voltage between two conductors The UL standard requires voltage at which isolation is not destroyed by application of AC for one minute. * Products with isolation voltage ranging from 2,500 Vrms to 5,000 Vrms are mainstream.

Types of Photocouplers (Packages)

Creepage distance

Clearance

Isolation voltage

AC 1 minute

DIP types

SMD types

SO6L ‧ Creepage distance≥8 mm, isolation thickness ≥0.4 mm

SO16L

VSON4 ‧ Ultra small ‧ Leadless SMD

‧ General-purpose package ‧ Supporting surface mount lead bend

‧Creeepage distance, clearance ≥7 mm, insulation thickness ≥0.4 mm ‧ Surface mount 6 leads (lead pitch =1.27 mm)

‧Creeepage distance, clearance ≥5 mm, insulation thickness ≥0.4 mm ‧ Surface mount 5 leads (lead pitch =1.27 mm) ‧Thin

‧ Surface mount 8 leads (lead pitch =1.27 mm)

‧ Surface mount 4 leads (lead pitch =1.27 mm) ‧ Surface mount 16 leads (lead pitch =1.27 mm)

‧ Creepage distance ≥8 mm, isolation thickness ≥0.4 mm ‧ SMD 16 leads (lead pitch=1.27 mm)

‧ Surface mount (lead pitch =1.27 mm)




SO16L

VSON4

Insulation thickness


Transmissive type in single mold Reflective type

Transmissive type in single mold with film

There are various types of internal package structures for photocouplers, due to restrictions related to required isolation performance, package size, inner chip size, etc. Transmissive type in single mold: Frame-mounted LED and frame-mounted photodetector face each other in the same molded package. Silicon resin is used for the optical transmission part between LED and photodetector.

With film: To raise isolation voltage, polyimide film is inserted between LED and photodetector.

Transmissive type in double mold: For this tansmissive structure, inner mold is white, and outer mold is black. Resin with high infrared light transmittance is used for white mold of the optical transmission part.

Reflective type: Frame-mounted LED and frame-mounted photodetector are on the same plane. LED light reflected in silicon resin reaches the photodetector. Thus, it is called reflective type.

Types of Photocouplers (Internal Structure)

Transmissive type in double mold

Photodetector Chip Photodetector Chip

Photodetector Chip Photodetector Chip


Major safety standards Parts standards UL1577 (cUL) Standard for isolation voltage (1 min) Approval organization: UL (Underwriters Laboratories Inc.) EN60747-5-5 Standard for maximum operating isolation voltage and maximum

overvoltage Approval organization: VDE (Verband Deutscher Elektrotechniker, Association for Electrical, Electronic & Information Technologies

(Germany) (e.g. VDE0844 Standard)) Approval organization: TUV (Technischer Uberwachungs Verein, Technical Inspection Association (Germany)) Set standards (Approval organization=BSI (UK), SEMKO (Sweden), etc.) EN60950 Standard for telecommunication network equipment (workstation, PC, printer, fax resistor, modem, etc.) EN60065 Standard of home appliance (TV, radio, VTR, etc.) equipment. Requirements for creepage distance, clearance and isolation thickness

When mounted in electrical equipment as a means of isolation to protect the human body from electric shock, various aspects of photocouplers are subject to safety standards. Regulations to ensure safety and various items related to electrical characteristics are standardized. These standards can be classified into “set standards” and “parts standards” from designers’ and manufacturers’ viewpoints. Set standards are the basis for designing and manufacturing equipment such as TVs, VTRs, and power source units. Set standards differ according to equipment type, isolation method and its class, driving voltage, etc. And, items (minimum isolation voltage, minimum creepage distance, minimum clearance, etc.) that must be ensured for isolation parts are defined as parts standards.

Safety Standards of Photocouplers


IF＝Input LED

current

IC＝Output collector current

E.g.） When IF=5mA is input, IC =10mA is obtained. CTR= IC /IF = 10 mA/5 mA×100 = 200%

Reference： hFE of transistor hFE(DC current gain) =Collector current(IC) /base current(IB)

Base current

Collector current

Current transfer ratio of transistor coupler: Current transfer ratio of a transistor coupler is expressed by amplification ratio of input current to output current, in the same manner as for hFE of a transistor.

Transfer ratio=CTR=Current Transfer Ratio=IC/IE =Output(collector) current/input current ×100(%)

Principal Characteristics of Photocouplers (Current Transfer Ratio: CTR)


A

Current meter Input LED Current IF

Circuit current

Trigger LED current is an important item for circuit design and life design.

Trigger LED current For logic output IC couplers, photorelays, triac output couplers, etc., trigger LED current is specified. “Trigger LED current” means “LED current that triggers change in the status”. (“LED current necessary to change output status”) IFT, IFH, IFLH, IFLH, etc., are used as symbols. Trigger LED current specified on the datasheet is the trigger LED current guaranteed for the product. Therefore, designers need to make sure that LED current is at least the trigger LED current (maximum) specified on the datasheet.

E.g.) When IF increases gradually from 0 mA to 5 mA for input, upon the transition from output off status to on status, IFT is 5 mA.

Input LED current

Circ

uit c

urre

nt

Measured trigger LED current

Maximum guaranteed trigger LED current of the

product

Principal Characteristics of Photocouplers (Trigger LED Current)


Aging variation data of photocouplers Optical output of LED deteriorates with the passage of time (optical output becomes smaller). In photocouplers, aging variation of optical output of LED is more dominant than that of optical detectors. So, aging variation is estimated by using data of aging variation of the adopted LED. In designing circuits, initial forward current(IF) is reflected by calculating output variation of LED from the usage environment of the set or total operating time. * For example, when duty (time duration of light emission) is 50% and working hours are 1,000 h, total operating time is calculated to be 500 hours.

The left-hand figure shows aging variation data of LED optical output. IF and Ta are parameters. The right-hand figure shows the operating time when LED optical output drops below a certain criterion. For example, point A in the left-hand figure and point B in the right-hand show aging variation under the same conditions (IF=50 mA, Ta=40℃, 8000 h).

Example of aging variation of optical output of GaAs

Point A: Dropped to 70% of initial value at about 8000 h.

Point B

Aging Variation Data of Photocouplers

Test conditions: IF=50 mA, Ta=40℃

Opt

ical

out

put P

o re

lativ

e ch

ange

rat

io(%

)

Test time (h)

Estimated F50% life

Estimated F0.1% life

Breakdown criterion: optical output ΔPo <-30%

Environment temperature(℃)

Estim

ated

life

time(

h)


Note: In the figure on the left, output signal is inverted. But the use of an emitter follower, which is shown below, can obtain the same output logic as input.

フォトトランジスタカプラの入力電流と出力電流

10V

Output

5V

RL RIN

0V

5V 10V

Output RL

10V

0V

Design example of signal interface by phototransistor coupler The figure below shows an interface circuit for transferring 5 V DC signals to a 10 V DC system. How should we design resistor of input LED RIN and resistor of output RL? And how should we select CTR of the phototransistor coupler? From the next page onward, we will study the following steps: Step 1. Design LED input current IF and input resistor RIN. Step 2. Calculate output current of phototransistor from IF and CTR. Step 3. Design output resistor RL. Step 4. Check each constant.

0V

10V

Interface circuit of 5 V DC and 10 V DC

Photocoupler Design Example (1)


What is the value of the photocoupler’s input current? It is decided by (1) power source voltage(5 V), (2) current limiting resistor(RIN) and (3) forward voltage of LED(VF) Decide current limiting resistor and input current(IF) from the specification example.

Forward voltage characteristic IF-VF characteristic changes by temperature.

Designers have to take temperature range into consideration. 10V

Output

5V

RL RIN

0V

5V IF

VF

（Specification example）

Ω 385mA 10

V) 1.15V (5

I

)VV (5R

F

FIN =

−=

−= IF becomes 10 mA,

if RIN is 385 Ω

Step 1. Design LED input current and input resistor RIN.


Forw

ard

Curr

ent

Forward Voltage

Forward Voltage


What is the value of the photocoupler’s output current(IC)? Because IF is decided, output current(IC) is calculated from current transfer ratio (IC/IF). Then, IC change ratio is assumed to be two. In the case of Rank GR (100% to 300%), IC=5 mA(IF)×2(IC change ratio) ×100% to 300%=10 mA to 30 mA Generally, guaranteed condition of transfer ratio of transistor coupler is VCE=5V. In actual use, choose RL to saturate VCE.

Current transfer ratio (CTR) rank Transistor photocouplers are classified by CTR.

10V

Output

5V

RL

385Ω

0V

5V 10mA IC

Step 2. Calculate output current of the phototransistor from IF and CTR.



Collector-emitter voltage VCE (V)

Colle

ctor

cur

rent

IC

(mA

)

10

20

1 kΩ

2 kΩ

0.5 kΩ

CTR=150%

CTR=100% VCE=Vcc-Ic×RL

10V

Output RL

VCE

0

LRVcc

0 5 10

Output characteristics and load line

IF=10 mA

IF=20 mA

Step 3. Design output resistor RL. Decide RL from IC-VCE characteristic of output transistor. To use it as signal transmitter, output transistor must be completely ON status (saturated).

When IF is 10 mA, in the case of RL=0.5 kΩ, VCE is 5 V and it is unsaturated but in the case of RL=2 kΩ , VCE < 0.2 V and it is saturated. So, we select 2 kΩ for RL.

About 0.1 V


Collector-emitter voltage

Colle

ctor

cur

rent


Confirmation of lifetime of the set It is necessary to confirm that the lifetime of the photocoupler satisfies that of the set in which the photocoupler is applied. Lifetime of the photocoupler can be calculated based on input current and environment temperature.

Temperature range ⇒ VF, CTR, allowable current, etc. Load resistance ⇒ switching speed, influence of dark current, etc.

10 V

Output

5 V

2 kΩ 385 Ω

0V

5 V 10 mA About 5 mA

CTR= 100 to 300%

Allowable current

Switching time -RL

CTR-Ta Dark current

Step 4. Check each constant. Consider whether there is sufficient margin for operating temperature, speed, lifetime design, tolerance of resistor, etc.



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[Chapter V] Basic Knowledge of Discrete Semiconductor Devices

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